Solar cell and solar cell module

ABSTRACT

A solar cell having a P-type silicon substrate where one main surface is a light-receiving surface and another is a backside, a plurality of back surface electrodes formed on a part of the backside, an N-type layer in at least a part of the light-receiving surface, and contact areas in which the substrate contacts the electrodes. The P-type silicon substrate is a silicon substrate doped with gallium and has a resistivity of 2.5 Ω·cm or less; and a back surface electrode pitch P rm  [mm] of contact areas in which the P-type silicon substrate is in contact with the back surface electrodes and the resistivity R sub  [Ω·cm] of the substrate satisfy the relation represented by the following formula (1). 
       log( R   sub )≤−log( P   rm )+1.0  (1)

This is a Continuation of application Ser. No. 16/731,132 filed Dec. 31,2019, which in turn is a Continuation of application Ser. No. 16/273,497filed Feb. 12, 2019, which in turn is a Continuation of application Ser.No. 15/523,923 filed May 2, 2017, which in turn is a national stage ofPCT/JP2015/005190 filed Oct. 14, 2015, which claims priority to JP2014-230517 filed Nov. 13, 2014. The disclosure of the priorapplications is hereby incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to a solar cell and a solar cell module.

BACKGROUND ART

Semiconductor substrates for solar cells are usually manufactured by aCzochralski method (CZ method), which can produce a single crystal witha large-diameter at relatively low cost. For example, P-typesemiconductor substrates can be obtained by drawing a silicon singlecrystal doped with boron by a CZ method, and slicing this singlecrystal.

Single crystal silicon solar cells (solar cells made of single crystalsilicon substrates) have been configured to have a structure in whichthe whole surface of the backside (the surface opposite to thelight-receiving surface) is in contact with the electrode via a BackSurface Field (BSF) structure.

The BSF structure can be easily introduced by a screen printing method,and is widely spread to be the mainstream of structures of presentsilicon single crystal solar cells.

To the BSF structure, a Passivated Emitter and Rear Contact Solar Cell(PERC) structure and a Passivated Emitter and Rear Locally DiffusedSolar Cell (PERL) structure come to be introduced in order to furtherimprove the efficiency.

The PERC structure and the PERL structure are methods to aggressivelyreduce the recombination rate of minority carriers on the back surface,that is, methods to reduce an effective surface recombination velocityon the back surface.

The cross section of a previous solar cell having a PERC structure isschematically shown in FIG. 9. As shown in FIG. 9, the solar cell 110 isprovided with the N-type layer 112 on the side of the light-receivingsurface of the silicon substrate 113 doped with boron (hereinafter, alsodescribed as a boron-doped substrate), and the finger electrodes 111 onthis N-type layer 112. In many cases, the solar cells have thepassivation layer 115 on the light-receiving surface. The solar cell isalso provided with the passivation layer 116 on the back surface, theelectrodes 114 on the back surface, and the contact areas 117 where theboron-doped substrate 113 is locally in contact with the back surfaceelectrodes 114.

The cross section of a previous solar cell having a PERL structure isschematically shown in FIG. 10. As shown in FIG. 10, the solar cell 110′is the solar cell 110 that is provided with the P⁺ layer (i.e., thelayer which is doped with P-type dopant in higher concentration than thesurrounding area (P-type silicon substrate)) 119 immediately under theback surface electrodes 114. This may be provided with N⁺ layer (i.e.,the layer which is doped with N-type dopant in higher concentration thanthe surrounding N-type layer 112) 118 under the light-receiving surfaceelectrode 111. Other structures are similar to the solar cell having aPERC structure in FIG. 9, and the explanation is omitted.

CITATION LIST Patent Literature

Patent Literature 1: pamphlet of International Patent Laid-OpenPublication No. WO 2000/073542

SUMMARY OF INVENTION Technical Problem

Even though a solar cell has the PERC structure or the PERL structure toreduce the recombination rate of minority carriers on the back surface,when the substrate is a boron-doped substrate, the interstitial boronatom combines with interstitial oxygen atoms by irradiated light to forma recombination site in the substrate bulk, which reduces the lifetimeof the minority carriers to degrade the characteristics of the solarcell. This phenomenon is also referred to as light-induced degradationof a solar cell using a boron-doped substrate.

In solar cells having the PERC structure and the PERL structure, theelectrode on the back surface is localized. This generates currentcrowding in the vicinity of the contact (i.e., the contact area wherethe substrate is in contact with the back surface electrode), and tendsto cause resistance loss. Accordingly, a substrate with low resistanceis preferable in the solar cell having the PERC structure or the PERLstructure. However, when using a substrate with low resistance, that is,in a situation in which more boron atoms are contained, the combinationof a boron atom and oxygen atoms increases to make the degradation(light-induced degradation) noticeable thereby.

On the other hand, using a substrate with higher resistance reduces thedegradation. In a solar cell having the PERC structure or the PERLstructure, however, current crowding significantly generates in thevicinity of the contact on the back surface to cause resistance loss asdescribed above. As a result, the characteristics degrade also in thiscase.

To eliminate the light-induced degradation, Patent Literature 1 suggeststhe use of gallium as P-type dopant instead of boron. It has beenimpossible to sufficiently prevent the resistance loss, however, only byusing a silicon substrate doped with gallium (hereinafter, also referredto as a gallium-doped substrate) as the substrate of the solar cellhaving the PERC structure or the PERL structure.

The present invention was accomplished in view of the above-describedproblems. It is an object of the present invention to provide a solarcell and a solar cell module having excellent conversion efficiency withthe resistance loss being prevented, with the solar cell using asubstrate the light-induced degradation of which is eliminated.

Solution to Problem

To achieve the above-described object, the present invention provides asolar cell comprising a P-type silicon substrate in which one mainsurface is a light-receiving surface and another main surface is abackside, a plurality of back surface electrodes formed on a part of thebackside, an N-type layer in at least a part of the light-receivingsurface of the P-type silicon substrate, and contact areas in which theP-type silicon substrate is in contact with the back surface electrodes;

wherein the P-type silicon substrate is a silicon substrate doped withgallium;

the P-type silicon substrate has a resistivity of 2.5 Ω·cm or less; and

a back surface electrode pitch P_(rm) [mm] of the plurality of backsurface electrodes and the resistivity R_(sub) [Ω·cm] of the P-typesilicon substrate satisfy the relation represented by the followingformula (1)

log(R _(sub))≤−log(P _(rm))+1.0  (1).

In such a solar cell, since the substrate is a gallium-doped substrate,the light-induced degradation is eliminated. The substrate is also asubstrate with lower resistance, which hardly generates current crowdingin the contact area to scarcely cause resistance loss. The solar cellhas the PERC structure with a lower resistance substrate, and cansufficiently reduce the recombination rate of the minority carriers onthe back surface side. In addition to these structures, the pitch of theelectrodes on the back surface (hereinafter, also referred to as a backsurface electrode pitch) and the resistivity of the substrate satisfythe relation represented by the foregoing formula (1), which makes itpossible to minimize the resistance loss due to the current crowding andto further increase the output power.

It is preferable that the resistivity of the P-type silicon substrate be0.2 Ω·cm or more.

Such solar cells can generate current in virtually the same level evenwhen the solar cell module is composed of solar cells with differentresistivity. Accordingly, excess loss can be reduced when the solar cellmodule is fabricated using such solar cells.

It is also preferable that the pitch of back surface electrodes be 10 mmor less.

Such a solar cell can be definitely a solar cell with excellentconversion efficiency.

It is also preferable that each of the contact areas have a higherP-type dopant concentration than other area.

Such a solar cell with a PERL structure having a so-called P⁺ layer isexcellent in conversion efficiency.

It is also preferable that the total area of the contact areas be 20% orless of the total backside area.

Such a solar cell makes it possible to further reduce the carrierrecombination on the contact between the substrate and the electrodewhile making the contact resistance lower between the substrate and theelectrode.

The present invention also provides a solar cell module comprising theinventive solar cell.

In the inventive solar cell, the light-induced degradation and theresistance loss are eliminated, while the conversion efficiency isexcellent. Accordingly, in the solar cell module provided with theinventive solar cell, the light-induced degradation and the resistanceloss are eliminated, while the conversion efficiency is excellent.

Advantageous Effects of Invention

In the inventive solar cell, since the substrate is a gallium-dopedsubstrate, the light-induced degradation is eliminated. The substrate isalso a substrate with lower resistance, which hardly generates currentcrowding near the contact area to scarcely cause resistance loss. Thesolar cell has the PERC structure or the PERL structure with a lowerresistance substrate, and can sufficiently reduce the recombination rateof the minority carriers on the back surface side. In addition to thesestructures, the electrode pitch on the back surface and the resistivityof the substrate satisfy the relation represented by the foregoingformula (1), which makes it possible to minimize the resistance loss dueto the current crowding and to further increase the output power.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view showing an example of the inventive solarcell;

FIG. 2 is a schematic drawing of the solar cell shown in FIG. 1 on aperspective projection of the P-type silicon substrate;

FIG. 3 is a sectional view showing an example of the inventive solarcell;

FIG. 4 is a schematic drawing of the solar cell shown in FIG. 3 on aperspective projection of the P-type silicon substrate;

FIG. 5 is a flow diagram of an example of a method for manufacturing theinventive solar cell;

FIG. 6 is a diagram of the average conversion efficiency of the solarcell as a function of the electrode pitch on the back surface and thesubstrate resistivity of the gallium-doped substrate after sufficientirradiation of the sunlight;

FIG. 7 is a diagram of the average conversion efficiency of the solarcell as a function of the electrode pitch on the back surface and thesubstrate resistivity of the boron-doped substrate after sufficientirradiation of the sunlight;

FIG. 8 is a diagram of the average short-circuit current density of thesolar cell as a function of the electrode pitch on the back surface andthe substrate resistivity of the gallium-doped substrate;

FIG. 9 is a sectional view schematically showing a previous solar cellhaving a PERC structure; and

FIG. 10 is a sectional view schematically showing a previous solar cellhaving a PERL structure.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the present invention will be more specifically described.

As described above, an excellent conversion efficiency solar cell witheliminating the resistance loss has been demanded for solar cells usinga substrate which eliminates the light-induced degradation. As astructure that can enhance the conversion efficiency, the PERC structureand the PERL structure have been proposed. However, only by using agallium-doped substrate which can eliminate light-induced degradationfor a solar cell having the PERC structure or the PERL structure, it hasbeen impossible to sufficiently prevent resistance loss to provide thesolar cell with excellent conversion efficiency.

The inventor has diligently investigated to solve the problems. As aresult, the inventor has found that the foregoing problem can be solvedwith the solar cell having a PERC structure or a PERL structure providedwith a gallium-doped substrate having a lower resistance, with the backsurface electrode pitch and the resistivity substrate satisfying aspecific relation; thereby brought the inventive solar cell and thesolar cell module to completion.

Hereinafter, an embodiment of the present invention will be specificallydescribed with reference to FIGS, but the present invention is notlimited thereto.

[Solar Cell]

FIG. 1 is a sectional view showing an example of the inventive solarcell. FIG. 2 is a schematic drawing of the solar cell shown in FIG. 1 ona perspective projection of the P-type silicon substrate 13. As shown inFIG. 1 and FIG. 2, the inventive solar cell 10 is provided with theP-type silicon substrate (gallium-doped substrate) 13 in which one ofthe main surface is a light-receiving surface and another main surfaceis a backside. This also has a plurality of back surface electrodes 14formed on a part of the backside of the P-type silicon substrate 13. TheP-type silicon substrate 13 has an N-type layer 12 in at least a part ofthe light-receiving surface, and also has the contact areas 17 where theP-type silicon substrate 13 is in contact with the back surfaceelectrodes 14. The N-type layer 12 is generally provided with thelight-receiving surface electrode 11 thereon.

In the present invention, the P-type silicon substrate 13 is agallium-doped substrate. By changing the P-type dopant from boron togallium like this, the light-induced degradation can be eliminated. TheP-type silicon substrate 13 has a resistivity (specific resistance) of2.5 Ω·cm or less. The resistivity more than 2.5 Ω·cm can cause currentcrowding in the vicinity of portions on the back surface side where theP-type silicon substrate 13 is in contact with the back surfaceelectrodes 14, which can cause resistance loss.

As described above, the inventive solar cell is provided with agallium-doped substrate having lower resistance (i.e., a substrate withhigh gallium concentration). The solar cell having a PERC structure or aPERL structure is particularly excellent in conversion efficiency whenhaving a substrate with lower resistance. Accordingly, the inventivesolar cell is particularly excellent in conversion efficiency. Theinventive solar cell, having a gallium-doped substrate with lowerresistance, hardly yield light-induced degradation, which occurs in aboron-doped substrate with lower resistance (i.e., a substrate with highboron concentration), and can keep the high efficiency.

In the inventive solar cell 10, the back surface electrode pitch P_(rm)[mm] of the plurality of back surface electrodes 14 and the resistivityR_(sub) [Ω·cm] of the P-type silicon substrate 13 satisfy the relationrepresented by the following formula (1)

log(R _(sub))≤−log(P _(rm))+1.0  (1).

The back surface electrode pitch 20 is shown in FIG. 1. When the backsurface electrode pitch 20 and the resistivity of the gallium-dopedsubstrate 13 do not satisfy the relation represented by the formula (1),it is not possible to sufficiently prevent resistance loss due tocurrent crowding, causing the decrease in the output power. The relationof the formula (1) was obtained by the following computer simulation.

In the solar cell shown in FIG. 1, the conversion efficiency of thesolar cell was simulated by using a computer, as a function of theelectrode pitch on the back surface and the resistivity of thegallium-doped substrate. The results are shown in FIG. 6.

In the solar cell shown in FIG. 9, the conversion efficiency of thesolar cell was also simulated by using a computer, as a function of theelectrode pitch on the back surface and the resistivity of theboron-doped substrate. The results are shown in FIG. 7. In FIG. 6 andFIG. 7, the conversion efficiency is shown by the light and shade. It isalso shown by contour lines of conversion efficiency, each of whichlinks each point of combination of the resistivity of the substrate andthe back surface electrode pitch.

FIG. 6 is a diagram showing the average conversion efficiency of thesolar cell as a function of the electrode pitch on the back surface andthe substrate resistivity of the gallium-doped substrate aftersufficient irradiation of the sunlight. As shown in FIG. 6, when theresistivity of the gallium-doped substrate was 2.5 Ω·cm or less, and theback surface electrode pitch and the substrate resistivity of agallium-doped substrate satisfied the relation represented by theforegoing formula (1), the results of conversion efficiency wereexcellent. On the other hand, as shown in FIG. 6, the conversionefficiency suddenly dropped when the resistivity was more than 2.5 Ω·cmor when the formula (1) was not satisfied.

FIG. 7 is a diagram of the average conversion efficiency of the solarcell as a function of the electrode pitch on the back surface and thesubstrate resistivity of the boron-doped substrate after sufficientirradiation of the sunlight. As shown in FIG. 7, when a boron-dopedsubstrate was used, the conversion efficiency was inferior to the caseof using a gallium-doped substrate. This shows an influence oflight-induced degradation. In this instance, the conversion efficiencysuddenly dropped in some cases even when the back surface electrodepitch and the substrate resistivity of a boron-doped substrate satisfiedthe relation represented by the formula (1).

In the solar cell shown in FIG. 1, the short-circuit current density ofthe solar cell was simulated, as a function of the electrode pitch onthe back surface and the resistivity of the gallium-doped substrate. Theresults are shown in FIG. 8. FIG. 8 is a diagram showing the averageshort-circuit current density of the solar cell as a function of theelectrode pitch on the back surface and the substrate resistivity of thegallium-doped substrate. In FIG. 8, the short-circuit current densityare shown by the light and shade. They are also shown as contour linesof the short-circuit current density, each of which links each point ofcombination of the resistivity of the substrate and the back surfaceelectrode pitch. As shown in FIG. 8, when the resistivity of agallium-doped substrate was 2.5 Ω·cm or less, and the back surfaceelectrode pitch and the resistivity of the gallium-doped substratesatisfied the relation represented by the formula (1), the short-circuitcurrent density showed smaller variation than the substrate resistivity.

When the resistivity of a gallium-doped substrate was 0.2 Ω·cm or more,the short-circuit current density showed much smaller variation than thesubstrate resistivity. These results reveal that the solar cell whichhas a resistivity of 0.2 Ω·cm or more and 2.5 Ω·cm or less and satisfiesthe relation represented by the formula (1) shows similar current evenwhen the solar cells have resistivity variation. Accordingly, it turnedout that these solar cells can reduce excess loss when they aremodularized. As described above, it is preferable that the resistivityof the P-type silicon substrate (gallium-doped substrate) 13 be 0.2 Ω·cmor more.

The thickness of the P-type silicon substrate 13 is not particularlylimited, and can be a thickness of 100 to 200 μm, for example. The shapeand area of the main surface of the P-type silicon substrate 13 is notparticularly limited.

It is also preferable that the back surface electrode pitch 20 of theplurality of back surface electrodes be 10 mm or less. Such a solar cellis excellent in conversion efficiency as shown in FIG. 6. The lowerlimit of the back surface electrode pitch 20 is not particularlylimited, and can be 1 mm, for example.

It is also preferable that each P-type dopant concentration in thecontact areas 17 be higher than the P-type dopant concentration in anarea other than the contact areas 17. As an example of such a solar cellhaving a PERL structure, the solar cell shown in FIG. 3 and FIG. 4 canbe enumerated. FIG. 3 is a sectional view showing an example of theinventive solar cell. FIG. 4 is a schematic drawing of the solar cellshown in FIG. 3 on a perspective projection of the P-type siliconsubstrate 13. In the solar cell shown in FIG. 3 and FIG. 4, the samereference number is given to each of the same components as those in thesolar cell shown in FIG. 1, and the explanation is omitted. As shown inFIG. 3 and FIG. 4, the solar cell 10′ is the one in which the foregoingsolar cell 10 is provided with N⁺ layer 18 immediately under thelight-receiving surface electrode 11, and the P⁺ layer 19 immediatelyunder the back surface electrodes 14 (in the vicinity of the contactareas 17). Such a solar cell having a PERL structure can be a solar cellthat is excellent in conversion efficiency.

It is also preferable that the total area of the contact areas 17 be 20%or less on the basis of the whole of the backside. In such a solar cell,it is possible to further reduce the recombination of carriers due tothe contact between the substrate and the electrode while making thecontact resistance much lower between the substrate and the electrode.The lower limit of the total area of the contact areas 17 is notparticularly limited, and can be 5%, for example. The electrode widthsof the light-receiving surface electrode 11 and the back surfaceelectrodes 14 are not particularly limited, and can be 15 to 100 μm, forexample.

As shown in FIG. 1, the inventive solar cell 10 is generally providedwith the surface passivation layer 16 on the back surface. It is alsopossible to have the surface passivation layer 15 on the light-receivingsurface. The passivation layer 15 on the light-receiving surface canalso act as an anti-reflection film, etc. As these passivation layers,it is possible to use a silicon nitride layer (SiNx layer) and SiO₂layer, which can be fabricated by using a plasma CVD, and to use athermal oxide layer too. The effect on anti-reflection shows maximumvalue when the layer thickness of the anti-reflection film is 85 to 105nm, which is favorable.

It is also possible to have metal such as aluminum on the whole surfaceof the back surface passivation layer 16 to form a structure in whichthe plurality of back surface electrodes 14 are connected with eachother (i.e., a structure in which the back surface electrodes 14 areintegrated).

Illustrative examples of the N-type dopant contained in the N-type layer12 and the N⁺ layer 18 include P (phosphorus), Sb (antimony), As(arsenic), and Bi (bismuth). Illustrative examples of the P-type dopantcontained in the P⁺ layer 19 include B (boron), Ga (gallium), Al(aluminum), and In (indium).

[Solar Cell Module]

Subsequently, the inventive solar cell module will be described. Theinventive solar cell module is provided with the foregoing inventivesolar cell. Specifically, it can be formed by connecting a plurality ofthe arranged inventive solar cells in series by using an interconnector, for example. Various module structures can be applied withoutlimiting thereto. In such a solar cell module, the light-induceddegradation and resistance loss are eliminated, and the conversionefficiency is excellent.

[Method for Manufacturing Solar Cell]

Then, the method for manufacturing the inventive solar cell will bedescribed with reference to FIG. 5. FIG. 5 is a process flow diagramshowing an example of a method for manufacturing the inventive solarcell. The method described below is a typical example, and the methodfor manufacturing the inventive solar cell is not limited thereto.First, as shown in FIG. 5 (a), a gallium-dopes substrate with aresistivity of 2.5 Ω·cm or less is prepared for the P-type siliconsubstrate in which one main surface is a light-receiving surface andanother main surface is a backside.

It is preferable that the resistivity of the gallium-doped substrateprepared in the step (a) be 0.2 Ω·cm or more. When using a gallium-dopedsubstrate, since the segregation coefficient of gallium is relativelyhigh, the resistivity of an ingot grown by a CZ method differs by aboutsix times at the top and at the tail. In order to manufacture a solarcell at low cost, it is desirable to use each of these ingots entirelyone piece, and it is preferable that the difference of the resistivityof a substrate be considered in the design stage. By preparing agallium-doped substrate with a resistivity of 0.2 Ω·cm or more in thestep (a), it is possible to manufacture plural solar cells having a PERCstructure or a PERL structure which can show similar current even whenthese solar cells differ the resistivity by about six times, and toreduce excess loss when these solar cells are modularized. This makes itpossible to manufacture a solar cell module at lower cost. The methodfor measuring the resistivity of a gallium-doped substrate is notparticularly limited, and includes a four-point prove method, forexample.

The silicon single crystal from which the gallium-doped substrate issliced can be produced by a CZ method, for example, as described above.In this case, gallium and a polycrystalline material may be introducedinto a crucible in a lump to form a raw material melt. It is desirableto produce dopant by producing and pulverizing a silicon single crystaldoped with higher concentration of gallium, and to adjust theconcentration by introducing the dopant into melted polycrystallinesilicon for CZ material so as to have a desired concentration, since itis necessary to precisely adjust the concentration, particularly in massproduction. The gallium-doped substrate can be obtained by slicing thusobtained gallium-doped silicon single crystal.

Subsequently, slice damages on the surface of the substrate can beremoved by etching with a high-concentration alkaline solution such assodium hydroxide and potassium hydroxide in a concentration of 5 to 60%,or mixed acid of hydrofluoric acid and nitric acid, etc. as shown inFIG. 5 (b).

Then, the substrate surface can be processed to form micro-roughnesscalled texture as shown in FIG. 5 (c). The texture is an effectivemethod to reduce the reflectance of a solar cell. The texture can beproduced by immersing the substrate in heated alkaline solution(concentration: 1 to 10%, temperature: 60 to 100° C.) such as sodiumhydroxide, potassium hydroxide, potassium carbonate, sodium carbonate,and sodium hydrogencarbonate for about 10 minutes to 30 minutes. In manycases, a certain amount of 2-propanol (IPA: isopropyl alcohol) is addedto the foregoing solution to promote the reaction.

After the damage-etching and texture formation, it is preferable toclean the substrate as shown in FIG. 5 (d). The cleaning can beperformed by using an aqueous acid solution of hydrochloric acid,sulfuric acid, nitric acid, hydrofluoric acid, or mixed solvent thereof;or pure water, for example.

Subsequently, as shown in FIG. 5 (e), N-type layer is formed on thelight-receiving surface of the gallium-doped substrate.

The method for forming an N-type layer in the step (e) is notparticularly restricted. For example, it is possible to enumerate amethod to thermally diffuse the dopant. This includes a vapor phasediffusion method in which POCl₃ (phosphoryl chloride) and the likeintroduced into a quartz tube furnace with carrier gas are diffused or acoating diffusion method in which a phosphorus-containing material andthe like applied onto a substrate is diffused by thermal treatment. Thecoating method in the coating diffusion method includes spin-coatingmethod, spray-coating method, ink-jet method, and screen printingmethod.

In the coating diffusion method, the N-type layer can be formed bycoating the light-receiving surface with a material which containsN-type dopant followed by thermal treatment. For the material whichcontains N-type dopant, it is possible to use a phosphorus diffusionsource, which turns to glass by thermal treatment. This phosphorusdiffusion source includes any known ones, and can be obtained by mixingP₂O₅, pure water, polyvinyl alcohol (PVA), and tetraethyl orthosilicate(TEOS), for example.

For a method for manufacturing a solar cell having a PERL structureprovided with an N⁺ layer on the light-receiving surface side and a Player on the back surface side, it is possible to enumerate a method inwhich the light-receiving surface is locally coated with the N-typedopant-containing material, and the back surface is locally coated withP-type dopant-containing material, and then the substrate is subjectedto a thermal treatment. In this case, it is possible to form diffusionmasks on the light-receiving surface and/or the back surface in order toprevent auto-doping and then to perform the thermal treatment in pluraltimes.

For the P-type dopant-containing material, it is possible to use a borondiffusion source, which turns to glass by thermal treatment. This borondiffusion source includes any known ones, and can be obtained by mixingB₂O₃, pure water, and PVA, for example.

Then, as shown in FIG. 5 (f), the PN junction is isolated by using aplasma etcher. In this process, samples are stacked so as to preventplasma and radical from getting into the light-receiving surface or thebackside, and the side faces of the substrate are dry-etched by severalmicrons under this stacked condition. This PN isolation by plasmaetching may be performed before removing the boron glass and phosphorusglass or may be performed after the removal. It is also possible to forma trench with laser as an alternative method to the PN isolation.

After the step (e), not a little quantity of glass layer is formed onthe surface of the substrate. The glass on the surface is removed byhydrofluoric acid, etc., as shown in FIG. 5 (g).

Subsequently, as shown in FIG. 5 (h), a surface passivation layer can beformed on the light-receiving surface of the gallium-doped substrate.For the light-receiving surface passivation layer, it is possible to usethe same ones described in the item of the solar cell.

Then, as shown in FIG. 5 (i), a surface passivation layer can bedeposited on the back surface of the gallium-doped substrate. For theback surface passivation layer, it is possible to use the same onesdescribed in the item of the solar cell.

Subsequently, as shown in FIG. 5 (j), the back surface passivation layercan be removed just on the contact area for a back surface electrode.The removal of the back surface passivation layer can be performed byusing a photo-lithography method or an etching paste, for example. Thisetching paste contains at least one selected from the group consistingof phosphoric acid, hydrofluoric acid, ammonium fluoride, and ammoniumhydrogen fluoride as an etching component together with water, organicsolvent, and a viscosity agent, for example.

At this stage, it is possible to determine a pitch to remove the backsurface passivation layer in the step (j) (which corresponds to an pitchof the contact areas) on the basis of the relation represented by theformula (1) for the resistivity of the gallium-doped substrate preparedin the step (a). This makes it possible to certainly manufacture a solarcell that is excellent in conversion efficiency. It is also possible toprecisely determine the upper and the lower limits of the back surfaceelectrode pitch P_(rm) [mm] and the resistivity R_(sub) [Ω·cm] tofabricate the solar cell.

Then, as shown in FIG. 5 (k), a paste for a back surface electrode isprinted onto the backside of the gallium-doped substrate, followed bydrying. For example, a paste in which Al powders are mixed with anorganic binder is applied onto the backside of the substrate by screenprinting. For a material of the back surface electrode, Ag and so onalso can be used. The back surface electrode needs to be formed on thecontact area at least. Alternatively, the back surface electrode may beformed and integrated to the whole back surface. In this case, the backsurface electrode actually contacts the substrate locally.

Subsequently, as shown in FIG. 5 (l), a paste for a light-receivingsurface electrode is printed onto the light-receiving surface of thegallium-doped substrate, followed by drying. For example, an Ag paste inwhich Ag powders and glass frit are mixed with an organic binder isapplied onto the light-receiving surface of the substrate by screenprinting. The step (k) and the step (l) may be performed in the oppositeorder.

After the foregoing printing of the electrodes, for the paste on alight-receiving surface electrode and the paste on a back surfaceelectrode, firing is done as shown in FIG. 5 (m). In this way, thelight-receiving surface electrode and the back surface electrode areformed by printing the pastes followed by firing. The firing isgenerally performed by thermal treatment at a temperature of 700 to 800°C. for 5 to 30 minutes. This thermal treatment makes the Ag powderspenetrate to the passivation layer (fire through) on the light-receivingsurface side to electrically conduct the light-receiving surfaceelectrode and the gallium-doped substrate. It is also possible to bakeback surface electrode and the light-receiving surface electrodeseparately.

In such a process, the solar cell shown in FIG. 1 and FIG. 3 can bemanufactured. The inventive solar cell module can be obtained bymodularizing the solar cells thus obtained.

It is to be noted that the present invention is not limited to theforegoing embodiment. The embodiment is just an exemplification, and anyexamples that have substantially the same feature and demonstrate thesame functions and effects as those in the technical concept describedin claims of the present invention are included in the technical scopeof the present invention.

1. A solar cell comprising: a P-type silicon substrate in which one mainsurface is a light-receiving surface and another main surface is abackside; a back surface passivation layer provided on a part of thebackside of the P-type silicon substrate; a plurality of back surfaceelectrodes in contact with the P-type silicon substrate in a pluralityof contact areas in which the back surface passivation layer is notprovided; an N-type layer formed on at least a part of thelight-receiving surface of the P-type silicon substrate; alight-receiving surface passivation layer; and a light-receiving surfaceelectrode that is in electrical contact with the N-type layer, wherein:the P-type silicon substrate is a silicon substrate doped with gallium,the P-type silicon substrate has a resistivity of 0.2 to 2.5 Ω·cm, theplurality of contact areas has a pitch P_(rm)[mm] of 1 to 10 mm, and thepitch P_(rm) [mm] of the plurality of contact areas and the resistivityR_(sub) [Ω·cm] of the P-type silicon substrate satisfy the relationrepresented by the following formula (1)log(R _(sub))≤−log(P _(rm))+1.0  (1).
 2. A solar cell comprising: aP-type silicon substrate in which one main surface is a light-receivingsurface and another main surface is a backside; a back surfacepassivation layer provided on a part of the backside of the P-typesilicon substrate; a plurality of back surface electrodes in contactwith the P-type silicon substrate in a plurality of contact areas inwhich the back surface passivation layer is not provided, the pluralityof back surface electrodes being connected with each other so that theplurality of back surface electrodes are integrated and formed on awhole surface of the back surface passivation layer; an N-type layerformed on at least a part of the light-receiving surface of the P-typesilicon substrate; a light-receiving surface passivation layer; and alight-receiving surface electrode that is in electrical contact with theN-type layer, wherein: the P-type silicon substrate is a siliconsubstrate doped with gallium; the P-type silicon substrate has aresistivity of 0.2 to 2.5 Ω·cm; the plurality of contact areas has apitch P_(rm)[mm] of 1 to 10 mm; and the pitch P_(rm) [mm] of theplurality of contact areas and the resistivity R_(sub) [Ω·cm] of theP-type silicon substrate satisfy the relation represented by thefollowing formula (1)log(R _(sub))≤−log(P _(rm))+1.0  (1).
 3. The solar cell according toclaim 1, wherein the pitch of the plurality of contact areas is formedby removing the back surface passivation layer according to the relationrepresented by the formula (1).
 4. The solar cell according to claim 2,wherein the pitch of the plurality of contact areas is formed byremoving the back surface passivation layer in the relation representedby the formula (1).
 5. The solar cell according to claim 1, having aconversion efficiency of 20% or more.
 6. The solar cell according toclaim 2, having a conversion efficiency of 20% or more.
 7. The solarcell according to claim 1, wherein the light-receiving surface electrodeand the contact areas have a width of 15 to 100 μm.
 8. The solar cellaccording to claim 2, wherein the light-receiving surface electrode andthe contact areas have a width of 15 to 100 μm.
 9. The solar cellaccording to claim 3, wherein the light-receiving surface electrode andthe contact areas have a width of 15 to 100 μm.
 10. The solar cellaccording to claim 4, wherein the light-receiving surface electrode andthe contact areas have a width of 15 to 100 μm.
 11. The solar cellaccording to claim 5, wherein the light-receiving surface electrode andthe contact areas have a width of 15 to 100 μm.
 12. The solar cellaccording to claim 6, wherein the light-receiving surface electrode andthe contact areas have a width of 15 to 100 μm.
 13. The solar cellaccording to claim 1, wherein the total area of the plurality of contactareas is 5 to 20% on the basis of the whole of the backside.
 14. Thesolar cell according to claim 2, wherein the total area of the pluralityof contact areas is 5 to 20% on the basis of the whole of the backside.15. The solar cell according to claim 1, wherein each of the contactareas has a higher P-type dopant concentration than other areas.
 16. Thesolar cell according to claim 2, wherein each of the contact areas has ahigher P-type dopant concentration than other areas.
 17. A solar cellmodule comprising the solar cell according to claim
 1. 18. A solar cellmodule comprising the solar cell according to claim 2.